Dual tuned volume coils adapted to provide an end ring mode

ABSTRACT

A magnetic resonance coil includes parallel elongate conductive elements ( 32 ) arranged to define a cylinder, and end rings ( 34, 35 ) disposed at opposite ends of the parallel elongate conductive elements and oriented transverse to the parallel elongate conductive elements. The end rings are configured to support a sinusoidal  1 H or other first species magnetic resonance at a magnetic field strength. The end rings and the parallel elongate conductive elements are configured to cooperatively support a second species birdcage magnetic resonance at the same magnetic field strength, the second species being different from  1 H or other first species.

FIELD OF THE INVENTION

The following relates to the magnetic resonance arts. The followingfinds illustrative application to magnetic resonance imaging andspectroscopy, and is described with particular reference thereto.However, the following will find application in other magnetic resonanceand radio frequency applications.

BACKGROUND OF THE INVENTION

Multinuclear magnetic resonance imaging and spectroscopy is of interestfor diverse applications, such as metabolic monitoring, diagnosis andclinical monitoring, and so forth. In some multinuclear applications,magnetic resonance excitation, magnetic resonance reception, or both areperformed at the ¹H magnetic resonance frequency and at a magneticresonance frequency of a second nuclear species such as ¹³C, ³¹P, or²³Na.

To enable simultaneous or concurrent operation at both the ¹H magneticresonance frequency and at a second species magnetic resonancefrequency, two separate, differently-tuned coils can be used. Thisenables true simultaneous operation at both magnetic resonancefrequencies, but has certain disadvantages. The two different magneticresonance coils occupy valuable bore space. Additionally, the two coilsmust be spatially aligned with each other, and within the scannerimaging volume, prior to the multinuclear magnetic resonance session.

Another approach is to use a single coil configured to operate at boththe ¹H magnetic resonance frequency and the magnetic resonance frequencyof a second species (also referred to herein as second species magneticresonance frequency). A transverse electromagnetic (TEM) volume coil canbe dual tuned by using interleaving coil elements (sometimes called coilrungs) for each resonance frequency. A birdcage volume coil can also bedouble tuned by using interleaving rungs together with radio frequency(RF) traps and a complex end ring arrangement. These approaches can moreefficiently utilize the bore space, and by using a single coil there isno need to spatially align two different coils prior to the multinuclearmagnetic resonance session. However, some disadvantages arise such asthe increased coil complexity and electrical coupling that may occurbetween the two resonance frequencies.

The following provides new and improved apparatuses and methods whichovercome the above-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with one aspect, a magnetic resonance coil is disclosed,comprising parallel elongate conductive elements arranged to define acylinder, and end rings disposed at opposite ends of the parallelelongate conductive elements and oriented transverse to the parallelelongate conductive elements. The end rings are configured to support asinusoidal ¹H magnetic resonance at a magnetic field strength. The coilis configured to support a second species magnetic resonance at the samemagnetic field strength, the second species being different from ¹H.Supporting a particular species magnetic resonance indicates thecapability to transmit radio-frequency signals and/or receive magneticresonance signals at the Larmor frequency of the particular nuclearspecies at the magnetic field strength.

In accordance with another aspect, a magnetic resonance scannercomprises a main magnet configured to generate a static (B₀) magneticfield (also called main magnetic field), magnetic field gradient coilsconfigured to superimpose selected magnetic field gradients on thestatic (B₀) magnetic field, and a magnetic resonance coil as set forthin the preceding paragraph.

In accordance with another aspect, a magnetic resonance coil isdisclosed, comprising parallel elongate conductive elements arranged todefine a cylinder, end rings disposed at opposite ends of the parallelelongate conductive elements and oriented transverse to the parallelelongate conductive elements, and a radio frequency shield proximate atleast to the end rings. The end rings, parallel elongate conductiveelements, and radio frequency shield are configured to cooperativelysupport a sinusoidal end ring first species magnetic resonance on theend rings at a magnetic field strength and a second species birdcagemagnetic resonance at the same magnetic field strength.

In accordance with another aspect, a magnetic resonance scannercomprises a main magnet configured to generate a static (B₀) magneticfield, magnetic field gradient coils configured to superimpose selectedmagnetic field gradients on the static (B₀) magnetic field, and amagnetic resonance coil as set forth in the preceding paragraph.

In accordance with another aspect, a magnetic resonance coil isdisclosed, comprising parallel elongate conductive elements arranged todefine a cylinder, end rings disposed at opposite ends of the parallelelongate conductive elements and oriented transverse to the parallelelongate conductive elements, and radio frequency traps operativelycommunicating with the elongate conductive elements and tuned to a ¹Hmagnetic resonance frequency at a magnetic field strength so as tosuppress ¹H birdcage magnetic resonance on the magnetic resonance coilat the magnetic field strength.

In accordance with another aspect, a magnetic resonance scannercomprises a main magnet configured to generate a static (B₀) magneticfield, magnetic field gradient coils configured to superimpose selectedmagnetic field gradients on the static (B₀) magnetic field, and amagnetic resonance coil as set forth in the preceding paragraph.

In accordance with another aspect, a magnetic resonance method isdisclosed for concurrently exciting or detecting magnetic resonance oftwo different species in a common magnetic field using a coil having apair of end rings and a plurality of transverse elongate conductiveelements, the method comprising: operating the end rings in a sinusoidalmode to generate or detect currents flowing at a first species magneticresonance frequency in the end rings; and concurrently operating thecoil in a second mode to generate or detect currents concurrentlyflowing at a second species magnetic resonance frequency at least in thetransverse elongate conductive elements.

One advantage resides in providing a dual-tuned radio frequency coil formultinuclear magnetic resonance operations.

Another advantage resides in more efficient use of bore space.

Another advantage resides in reduced complexity of a dual-tuned radiofrequency coil for multinuclear magnetic resonance operations.

Another advantage resides in facilitating simultaneous operation of adual-tuned coil at ¹H and second species magnetic resonance frequencies.

Still further advantages of the present invention will be appreciated tothose of ordinary skill in the art upon reading and understand thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will be described in detail hereinafter, by wayof example, on the basis of the following embodiments, with reference tothe accompanying drawings, wherein:

FIG. 1 diagrammatically shows a system for performing multinuclearmagnetic resonance imaging or spectroscopy;

FIG. 2 diagrammatically shows a dual-tuned radio frequency coil suitablefor use in the system of FIG. 1;

FIG. 3 plots sinusoidal resonance frequency versus end ring radius foran end ring modeled as a continuous unshielded circular annularconductor without intervening capacitance or inductance elements;

FIG. 4 diagrammatically shows an electrical schematic for a suitable ¹Hradio frequency trap suitable for use in the coil of FIG. 2; and

FIG. 5 diagrammatically shows a dual-tuned radio frequency coil suitablefor use in the system of FIG. 1 and having a different radio frequencyshield or screen configuration as compared with the coil of FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to FIG. 1, a magnetic resonance scanner 10 includes amain magnet 12 generating a static (B₀) magnetic field in an examinationregion 14 in which is disposed a subject 16 (shown in dashed lines inFIG. 1). The illustrated magnetic resonance scanner 10 is a horizontalbore-type scanner shown in cross-section to reveal selected componentsfor illustration. The magnetic resonance scanner 10 is a high-fieldscanner in which the main magnet 12 produces the static (B₀) magneticfield in the examination region 14 at a magnetic field strength greaterthan 3 Tesla, and in some embodiments greater than or about 5 Tesla. Insome embodiments, the main magnet 12 produces a static (B₀) magneticfield in the examination region 14 at a magnetic field strength of 7Tesla. Higher magnetic field strengths are also contemplated.

The magnetic resonance scanner 10 also includes magnetic field gradientcoils 18 that superimpose selected magnetic field gradients on thestatic (B₀) magnetic field to perform various tasks such as spatiallyrestricting magnetic resonance excitation, spatially encoding magneticresonance frequency and/or phase, spoiling magnetic resonance, or soforth. Optionally, the magnetic resonance scanner may include otherelements not shown in FIG. 1, such as a bore liner, active coil orpassive ferromagnetic shims, or so forth. The subject 16 is suitablyprepared by being placed on a movable subject support 20 which is theninserted along with the supported subject 16 into the illustratedposition for magnetic resonance acquisition. For example, the subjectsupport 20 may be a pallet or table that is initially disposed on acouch 22 adjacent the magnetic resonance scanner 10, the subject 16placed onto the support 20 and then slidably transferred from the couch22 into the bore of the magnetic resonance scanner 10.

With continuing reference to FIG. 1 and with further reference to FIG.2, a magnetic resonance coil 30 is provided to excite and receivemagnetic resonance. In multinuclear magnetic resonance, two or morenuclear species are of interest, such as two or more nuclear speciesselected from a group consisting of ¹H, ¹³C, ³¹P, and ²³Na. In somemultinuclear magnetic resonance applications, two species are ofinterest, namely ¹H and a second nuclear species other than ¹H, such as¹³C, ³¹P, ²³Na, or so forth.

The magnetic resonance coil 30 has a birdcage configuration including aplurality of parallel elongate conductive elements 32 (sometimes called“rungs” 32 herein) arranged to define a cylinder, and end rings 34, 35disposed at opposite ends of the parallel elongate conductive elements32 and oriented transverse to the parallel elongate conductive elements32. A generally cylindrical radio frequency shield 36 surrounds theparallel elongate conductive rungs 32 and generally coaxial with thecylinder defined by the parallel elongate conductive elements 32. Theradio frequency shield 36 includes annular flanges 38, 39 disposedparallel with and proximate to respective end rings 34, 35 at oppositeends of the parallel rungs 32. The illustrated magnetic resonance coil30 is a whole-body coil, sized to fit coaxially into the cylindricalbore of the illustrated horizontal bore scanner 10; however, themagnetic resonance coil can also be sized as a head coil to fit over thehead of the subject 16, or sized as a limb coil to fit over an arm orleg of the subject 16, or so forth.

The magnetic resonance coil 30 is a dual-tuned radio frequency coilsupporting end ring resonance at a first magnetic resonance frequency ofa first nuclear species, and birdcage magnetic resonance at a secondmagnetic resonance frequency of a second nuclear species different fromthe first nuclear species. In the following, the end ring resonance isassumed to correspond to the ¹H magnetic resonance frequency at amagnetic field strength of a static (B₀) magnetic field generated by themain magnet 12, while the birdcage resonance is assumed to correspond toa second species magnetic resonance frequency at the same magnetic fieldstrength, where the second species magnetic resonance frequency isdifferent from the ¹H magnetic resonance frequency. However, it is alsocontemplated for the end ring resonance to correspond to the magneticresonance frequency of another nuclear species besides ¹H at a magneticfield strength.

The birdcage coil 30 resonates as a volume resonator with a birdcageresonance at the second species magnetic resonance frequency.Optionally, the birdcage magnetic resonance frequency is tuned bysuitable tuning elements in the elongate conductive elements or rungs,such as illustrated by discrete rung capacitances 40, or by distributedcapacitance in the rungs 32, end rings 34, 35, or both, or by discreteor distributed inductances, or so forth. The use of multiple tuningcapacitances, or distributed capacitance, can be advantageous in orderto reduce high localized electric fields in the vicinity of the tuningcapacitors. In some embodiments, geometrical or material aspects of theshielding 36 and annular flanges 38, 39 such as but not limited tomaterial conductance, spacing from the rungs 32, thickness of the meshor screen material of the shielding, or so forth also affects thebirdcage magnetic resonance frequency.

With brief reference to FIG. 3, the end rings 34, 35 (shown in FIG. 2)are also configured to resonate sinusoidally at the ¹H magneticresonance frequency. FIG. 3 plots sinusoidal resonance frequency versusend ring radius for an end ring modeled as a continuous unshieldedcircular annular conductor without intervening capacitance or inductanceelements. (As used herein, the term “sinusoidal resonance” and the likeis intended to encompass sinusoidal resonance irrespective of phase, andencompasses, for example, what might also be termed “cosinusoidalresonance” depending upon the reference phase). The plot of FIG. 3 wasgenerated by electromagnetic simulation for radii up to 20 cm and thecurve is extrapolated to 30 cm radius. It is recognized herein that forhigh-field magnetic resonance and for an end ring 34, 35 of sufficientlylarge radius, the sinusoidal mode circulates at a useful frequency rangematching certain magnetic resonance frequencies of interest. Forinstance, the ¹H magnetic resonance frequency is 298 MHz in a static(B₀) magnetic field of 7 Tesla. As indicated in FIG. 3, the sinusoidalresonance of the end rings 34, 35 having reasonable radii of about 15centimeters, which is a typical radius for a human head coil, is closeto the ¹H magnetic resonance frequency at a magnetic field strength of 7Tesla. Taking into account the effect of the cylindrical shield 36 andadjacent shielding flanges 38, 39, the resonance frequency of thesinusoidal mode can be closely matched to 298 MHz in a head coilconfiguration. The shielding 36, 38, 39 also advantageously sharpens theresonance quality (Q-factor) of the sinusoidal resonance supported bythe end rings 34, 35.

With continuing reference to FIGS. 2 and 3, it can be seen that when theend rings 34, 35 have a radius of between about 10 centimeters and about20 centimeters, the resonance frequency for the sinusoidal mode isbetween about 200 MHz and about 500 MHz (taking into account the effectsof the shielding 36, 38, 39, and allowing for optional tuning by addingreactance elements such as capacitances or capacitive gaps in theannular conductor). These resonance frequencies span the magneticresonance frequencies of some of the nuclear species of interest at highmagnetic field. FIG. 3 also extrapolates the calculated curve out to 128MHz (extrapolation indicated by dashed lines), corresponding to a staticmagnetic field of about 3 Tesla. The extrapolation indicates thatunshielded and untuned end rings with diameters of about 60 centimeters(30 centimeters radius) to 70 centimeters (35 centimeters radius), whichis the typical diameter for a whole body radio frequency coil, supportsinusoidal resonance at about the ¹H proton magnetic resonance frequencyfor a magnetic field strength of 3 Tesla.

The plot of FIG. 3 is illustrative for unshielded continuous annularconductors. It is to be understood that the sinusoidal resonancefrequency supported by end rings 34, 35 of a given diameter can beadjusted over a substantial frequency range by inclusion of tuningelements, by the configuration of the shielding 36, 38, 39, by thethickness and width of the end rings 34, 35, and so forth. Thesinusoidal resonance frequency of the end rings 34, 35 can be tuned tothe ¹H magnetic resonance frequency or to another magnetic resonancefrequency of interest by adding lumped or distributed capacitances orinductances along the end rings, by varying parameters such as theradius, the thickness or other cross-sectional dimensions of the endrings 34, 35, by adjusting the shielding 36, 38, 39, by adding reactanceelements such as capacitances or capacitive gaps in the end rings 34,35, by adding dielectric materials between the end ring 34 and flange38, and/or end ring 35 and flange 39, or by various combinations of suchadjustments. Moreover, it is recognized herein that at higher magneticfield, the spatial uniformity provided by sinusoidal resonance in theend rings 34, 35 is largely determined by the dielectric and conductivecharacteristics of the subject 16 or other loading of the coil 30;hence, at static B₀ magnetic field values greater than or about 3 Tesla,the relatively large unloaded non-uniformity of the B₁ field generatedby the sinusoidal mode is acceptable.

With reference back to FIG. 2, the end rings 34, 35 are connected to therungs 32. The rungs 32 interfere with the sinusoidal end ring resonance.To reduce or eliminate such interference, radio frequency traps 44, 45are suitably disposed with or integrated into the rungs 32. The traps44, 45 are RF filters designed to present a blocking high impedance atthe sinusoidal resonance frequency supported by the end rings 34, 35,while having almost no effect on the birdcage resonance at a secondfrequency different from the resonance frequency supported by the endrings 34, 35. The traps 44, 45 virtually isolate the end rings 34, 35from the rungs 32 at the end ring resonance. For example, if thedesigned magnetic field strength is 7 Tesla and end rings are designedto support the ¹H magnetic resonance frequency at 7 Tesla (i.e., 298MHz), then the radio frequency traps 44, 45 are suitably designed asnotch filters to block the 298 MHz resonance frequency. As illustratedin FIG. 2, in some embodiments the radio frequency traps 44, 45 aredisposed at ends of the rungs 32 close to the end rings 34, 35.

With reference to FIG. 4, in some embodiments the radio frequency traps44, 45 are parallel LC tank circuits (where L denotes inductance and Cdenotes capacitance) for which the impedance maximizes at a frequency of

$\frac{1}{2 \cdot \pi \cdot \sqrt{LC}}.$

Other radio frequency trap configurations are also contemplated. Withthe traps 44, 45 tuned to the ¹H magnetic resonance frequency, the traps44, 45 block current flow at the ¹H magnetic resonance frequency butallow current flow at other frequencies such as at the second speciesmagnetic resonance frequency at which the birdcage resonance modeoperates.

With reference to FIG. 5, a modified coil 30′ includes the rungs 32 andend-rings 34, 35. However, the shielding 36, 38, 39 of the coil of FIG.2 is replaced in the modified coil 30′ of FIG. 5 by an open shield 36′that does not include shielding material in a central region. In thiscase, the cylindrical shield 36′ is divided into two separated parts bythe open central region. At the birdcage resonance frequency, thebirdcage coil behavior is close to an unshielded birdcage, whichsubstantially improves coil sensitivity. The shielding further includesthe flanges 38, 39. Optionally, one flange, such as the flange 38, maybe replaced by an end cap 38′. Although not shown, such replacement of aflange by an end cap can also be made in the coil 30 of FIG. 2. The openshield 36′ advantageously increases coil sensitivity for the secondspecies (non-¹H) magnetic resonance, because radiation loss at thesecond species magnetic resonance frequency is not significant. The openshield 36′ does not adversely affect the coil sensitivity for the ¹Hmagnetic resonance because the sinusoidal resonance coupling with the ¹Hmagnetic resonance is supported by the end rings 34, 35 which arerelatively far away from the open central region of the open shield 36′.

Having described some illustrative coil embodiments 30, 30′, somefurther illustrative implementations are described by way of furtherexample.

The end rings 34, 35 are suitably tuned to a sinusoidal resonance modeat the ¹H magnetic resonance frequency by adjustable ring capacitors(not shown) or other elements affecting the sinusoidal resonance of theend rings 34, 35. In some embodiments in which a desired diameter of thering is pre-determined, individual inductors in series with ringcapacitors can be used to tune the end rings 34, 35 to the sinusoidalresonance mode at the ¹H magnetic resonance frequency. At the ¹Hmagnetic resonance frequency, the traps 44, 45 in the coil rungs 32 havehigh impedance which suppresses the current from flowing to the coilrungs 32. In the illustrated embodiments, the traps 44, 45 are locatedon or with the rungs 32 near the connections to the respective end rings34, 35. Thus the two end rings 34, 35 can be fed in quadrature fortransmitting and receiving of ¹H signal. At the second species (non-¹H)frequency, the traps 44, 45 function approximately as a short circuit,which allow the current at the second species magnetic resonancefrequency to flow between the end rings 34, 35 and the rungs 32 inaccordance with the birdcage resonance mode. The coil 30, 30′ thusdefines a shielded band-pass birdcage coil resonant at the secondspecies magnetic resonance frequency. The birdcage resonance can betuned to the desired second species magnetic resonance frequency byadjusting values of the rung capacitors 40. Optionally, the birdcageresonance frequency can also be adjusted by adjusting the diameters ofthe end rings 34, 35, adjusting the end ring positions along the rungs32, by including tuning end ring elements such as capacitors orinductors, or so forth. Where a parameter such as an end ring tuningcapacitor value affects both the sinusoidal and birdcage resonancefrequencies, the parameter values can be selected by iterativeadjustment in conjunction with suitable electromagnetic modeling to tuneboth the sinusoidal and birdcage resonance frequencies together.

To further illustrate advantages of the dual-tuned volume coilsdisclosed herein as compared with a TEM multi-nuclear coil, the coil 30of FIG. 1 was modeled as a head-size transmit/receive (T/R) coil with adiameter of 30 cm and rung lengths of 21 cm. The cylindrical shielddiameter was modeled as 35 cm and the shield length was modeled as 23cm. Twelve rungs 32 were included in the coil model. The two end rings34, 35 were modeled as flat annular rings with inner diameter of 28 cmand outer diameter of 31 cm. The end rings 34, 35 were tuned to the ¹Hmagnetic resonance frequency of 298 MHz (corresponding to a magneticfield strength of 7 Tesla), and the shielded birdcage coil was tuned tothe ³¹P frequency of 120.7 MHz for the same 7 Tesla magnetic fieldstrength. For comparison, a 12-element TEM coil was modeled with thesame size as the birdcage coil, and tuned to the same ³¹P frequency of120.7 MHz. A 20 cm-diameter spherical phantom (conductivity □=0.855 S/m,relative permittivity

=80) was used to model loading of both coil models. In the model, thecoil elements and the shield structure were separated by air.

The two end rings were modeled as operated in a two-port drive inquadrature at 298 MHz, where one port was fed in one end ring andanother port with opposite voltage but 90-degree out of phase is fed inthe other end ring. The birdcage coil was two-port driven in quadraturein the middle of two rungs at 120.7 MHz. The comparative TEM coil wasalso modeled as operated in a two-port drive in quadrature, acrosscapacitors at the ends. The |B₁ ⁺|-field (radio-frequency transmitfield) in three center slices of the sphere phantom were calculated atboth resonance frequencies, 298 MHz and 120.7 MHz. The transmitefficiency was calculated as

${\eta = \frac{{B_{1}^{+}}_{ave}}{\sqrt{P_{abs}}}},$

where |B₁ ⁺|_(ave) is the average |B₁ ⁺|-field in the center transverseslice of the sphere phantom and P_(abs) is the total absorbed power ofthe phantom. The coil sensitivity was calculated as |B₁ ⁺|_(ave) perunit current in the coil rungs (or ring in the case of end ring onlyresonance mode).

The |B₁ ⁺|-field uniformity at the ¹H magnetic resonance frequency wasfound to be dominated by the dielectric effect of the phantom material,which is comparable to a T/R birdcage or TEM volume coil. The |B₁⁺|-field uniformity at the ³¹P magnetic resonance frequency was found tobe relatively uniform and similar to that of a TEM coil. Table 1 liststhe calculated transmit efficiency and maximum local SAR (10 g phantommaterial averaged SAR) at |B₁ ⁺|_(ave)=1□ T. The coil sensitivity forthe modeled duakuned volume coil and for the comparative 12-element TEMvolume coil at 120.7 MHz is also given in Table 1. It is seen that, at120.7 MHz, the birdcage coil has about the same transmit efficiency asthe TEM coil, but has less local SAR and substantially higher coilsensitivity. Furthermore, the birdcage coil has a less complex structurewith only twelve rungs, whereas the dual tuned TEM volume coil employeda more complicated structure of twenty-four elements, of which twelveelements provided resonance at the ¹H magnetic resonance frequency andanother twelve interleaved elements provided resonance at the ³¹Pmagnetic resonance frequency.

TABLE 1 Dual-tuned volume coil Comparative Two 12-element end rings atBirdcage at TEM Volume coil 298 MHz 120.7 MHz at 120.7 MHz Transmitefficiency 0.5□ T/W^(1/2) 0.8□ T/W^(1/2) 0.8□ T/W^(1/2) Max. local SARat 2.5 W/kg 0.6 W/kg 0.8 W/kg |B₁ ⁺|_(ave) = 1□ T Coil sensitivity —2.5□ T/A 1.4□ T/A

Another advantage of the dual-tuned volume coil employing sinusoidalend-ring and birdcage resonances is that the coil sensitivity at thebirdcage resonance (i.e., second species magnetic resonance) can beenhanced by opening the shield in the middle, as shown in FIG. 5. Theopen shield 36′ of FIG. 5 is not compatible with a TEM coil because itwould not support the TEM resonance mode.

A modeling example of the modified coil 30′ of FIG. 5 is also presented.The same coil model as previously described was again used, except thatthe cylindrical shield was opened in the middle as shown in FIG. 5, withthe central open region being a 10 cm wide gap. The optional end cap 38′was not included in the modeling. Table 2 lists the calculated resultsfor the model with a closed shield (as in FIG. 2) and with a partiallyopen shield (as in FIG. 5). As seen in Table 2, the coil sensitivity isincreased from 2.5□ T/A for the coil with the closed shield to 6.4□ T/Afor the coil with the open shield having the 10 cm gap. The coilsensitivity is more than doubled by having the 10 cm gap. The high coilsensitivity of the open-shielded coil is not readily attainable indual-tuned coils for 7 Tesla operation that are shielded at both the ¹Hand second species magnetic resonance frequencies. While providing ashield for the ¹H coil resonance is advantageous at 7 Tesla to reduceradiation loss, providing a shield for the second species (i.e., non-¹H)coil resonance is not advantageous, because most non-¹H magneticresonance frequencies are substantially lower than the ¹H magneticresonance frequency (for the same magnetic field strength) andaccordingly exhibit substantially lower radiation loss. The partialshielding of the coil of FIG. 5 is enabled by the combination ofsinusoidal end ring resonance for the ¹H magnetic resonance coupling andbirdcage resonance for the second species magnetic resonance coupling.

TABLE 2 Two end rings Birdcage at 298 MHz at 120.7 MHz Transmitefficiency 0.5□ T/W^(1/2) 0.8□ T/W^(1/2) Max. local SAR at 2.8 W/kg 0.6W/kg |B₁ ⁺|_(ave) = 1□ T Coil sensitivity — 6.4□ T/A

Modeling was also performed to estimate peak electric fielddistributions for the dual-tuned (sinusoidal end ring/birdcage) coil 30′of FIG. 5 having a 10 cm gap in the shield 36′. The gap in the shield36′ was found to result in leakage of electromagnetic field outside thecoil which can increase radiation losses. However, this effect is notexpected to be problematic because a typical magnetic resonance scannerincludes another body-sized shield which could help contain the powerloss. Moreover, radiation loss for the 128 MHz ¹H magnetic resonance at3 Tesla is not problematic for birdcage type head T/R coils. At highermagnetic field strengths, a design tradeoff can be made betweenradiation losses (suppressed by reducing the gap of the shield 36′) andcoil sensitivity to the second species magnetic resonance (enhanced byincreasing the gap of the shield 36′).

In the illustrated embodiments, the coil has a birdcage configuration inwhich the end rings 34, 35 are operatively coupled with the parallelelongate conductive elements 32 to support the second species birdcagemagnetic resonance. This allows the option of using either the closedradio frequency shield 36 or the open radio frequency shield 36′. It isalso contemplated to operatively connect the parallel elongateconductive elements with the shield, which in such embodiments is aclosed shield similar to the radio frequency shield 36, such that thesecond species resonance is supported in a TEM mode while the end ringssupport only the sinusoidal first species (e.g., ¹H) magnetic resonance.In such embodiments, the radio frequency traps blocking ¹H (or otherfirst species) resonance on the parallel elongate conductive elements 32suppress inductive coupling at the ¹H frequency.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof. In the claims, anyreference signs placed between parentheses shall not be construed aslimiting the claim. The word “comprising” does not exclude the presenceof elements or steps other than those listed in a claim. The word “a” or“an” preceding an element does not exclude the presence of a pluralityof such elements. The disclosed method can be implemented by means ofhardware comprising several distinct elements, and by means of asuitably programmed computer. In the system claims enumerating severalmeans, several of these means can be embodied by one and the same itemof computer readable software or hardware. The mere fact that certainmeasures are recited in mutually different dependent claims does notindicate that a combination of these measures cannot be used toadvantage.

1. A magnetic resonance coil comprising: parallel elongate conductiveelements arranged to define a cylinder; and end rings disposed atopposite ends of the parallel elongate conductive elements and orientedtransverse to the parallel elongate conductive elements; the end ringsbeing configured to support a sinusoidal ¹H magnetic resonance at amagnetic field strength; and the coil being further configured tosupport a second species magnetic resonance at the same magnetic fieldstrength, the second species being different from ¹H.
 2. The magneticresonance coil as set forth in claim 1, wherein the end rings and theparallel elongate conductive elements cooperate to support the secondspecies magnetic resonance as a birdcage second species magneticresonance at the magnetic field strength.
 3. The magnetic resonance coilas set forth in claim 1, wherein the parallel elongate conductiveelements include radio frequency trap elements configured tosubstantially suppress ¹H magnetic resonance on the parallel elongateconductive elements at the magnetic field strength.
 4. The magneticresonance coil as set forth in claim 1, further comprising: one or moreradio frequency shield portions arranged proximate to at least the endrings, the one or more radio frequency shield portions cooperating withthe end rings to configure the end rings to support the sinusoidal ¹Hmagnetic resonance at the magnetic field strength.
 5. The magneticresonance coil as set forth in claim 4, wherein the one or more radiofrequency shield portions comprise: at least one of a shield flangeportion and a shield endcap portion arranged at each end of the parallelelongate conductive elements to shield a proximate one of the end rings.6. The magnetic resonance coil as set forth in claim 4, wherein the oneor more radio frequency shield portions comprise: a cylindrical radiofrequency shield further including at least one of a shield flangeportion and a shield endcap portion arranged at each end of the parallelelongate conductive elements to shield a proximate one of the end rings.7. The magnetic resonance coil as set forth in claim 6, wherein: the endrings and the parallel elongate conductive elements cooperate to supportthe second species magnetic resonance as a birdcage second speciesmagnetic resonance at the magnetic field strength; and the cylindricalradio frequency shield has a central open region.
 8. A magneticresonance scanner comprising: a main magnet configured to generate astatic (B₀) magnetic field; magnetic field gradient coils configured tosuperimpose selected magnetic field gradients on the static (B₀)magnetic field; and a magnetic resonance coil as set forth in claim 1.9. A magnetic resonance coil comprising: parallel elongate conductiveelements arranged to define a cylinder; end rings disposed at oppositeends of the parallel elongate conductive elements and orientedtransverse to the parallel elongate conductive elements; and a radiofrequency shield proximate at least to the end rings; the end rings,parallel elongate conductive elements, and radio frequency shield beingconfigured to cooperatively support a sinusoidal end ring first speciesmagnetic resonance on the end rings at a magnetic field strength and asecond species birdcage magnetic resonance at the same magnetic fieldstrength.
 10. The magnetic resonance coil as set forth in claim 9,wherein the parallel elongate conductive elements include radiofrequency traps tuned to block the first species magnetic resonancefrequency at the magnetic field strength.
 11. The magnetic resonancecoil as set forth in claim 9, wherein the radio frequency shieldcomprises: a flange or endcap disposed proximate to a first end ring ofthe end rings; and a flange or endcap disposed proximate to a second endring of the end rings.
 12. The magnetic resonance coil as set forth inclaim 11, wherein the radio frequency shield further comprises: acylindrical radio frequency shield surrounding the parallel elongateconductive elements and coaxial with the cylinder defined by theparallel elongate conductive elements.
 13. The magnetic resonance coilas set forth in claim 12, wherein the cylindrical radio frequency shieldhas an open central region.
 14. A magnetic resonance method forconcurrently exciting or detecting magnetic resonance of two differentspecies in a common magnetic field using a coil having a pair of endrings and a plurality of transverse elongate conductive elements, themethod comprising: operating the end rings in a sinusoidal mode togenerate or detect currents flowing at a first species magneticresonance frequency in the end rings; and concurrently operating thecoil in a second mode to generate or detect currents concurrentlyflowing at a second species magnetic resonance frequency at least in thetransverse elongate conductive elements.
 15. The magnetic resonancemethod as set forth in claim 14, wherein the operating of the coil inthe second mode comprises: operating the coil in a birdcage mode togenerate or detect currents concurrently flowing at the second speciesmagnetic resonance frequency in the transverse elongate conductiveelements and in the end rings.